The invention relates generally to plasma sputtering of materials. In particular, the invention relates to the target and associated magnetron creating a magnetic field to enhance sputtering.
Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconducting integrated circuits. In sputtering of metals and metal compounds, a metallic target is typically biased negatively to attract ions to the target at such high energies that the ions dislodge or sputter atoms from the target, and these sputtered atoms coat the wafer placed in opposition to the target. One of the most challenging applications of sputtering deposits metal into a narrow and deep hole, that is, one have a high aspect ratio. Such holes may be via holes connecting two levels of metallization through an intervening dielectric layer. In advanced circuitry, via holes may have aspect ratios of 5:1 or greater with via diameters of 0.18 xcexcm or less.
Sputtering is fundamentally a ballistic process ill suited to reach deep within high-aspect ratio holes. The sputtered metal tends to build up on the lip of the hole and bridge the hole before the hole is filled, thereby forming voids in the metallization within the hole.
Sputtering may be used in several applications. In one recently developed technology, copper is used as the metallization in place of aluminum to achieve several benefits. Copper has a lower electrical resistivity and is less prone to electromigration that is aluminum. Further, copper is easily and economically filled into high-aspect ratio hole by electro-chemical plating (ECP). However, prior to plating copper, the via hole needs to be lined with a thin copper seed layer both to initiate the plated layer and to act as an electrode in the electro-plating process. The seed layer requires only a few nanometers of copper, but the thickness needs to be fairly uniform from the top to the bottom of the via sidewall. Sputtering can also be used for depositing thin barrier layers into the via holes, for example, a barrier of Ta/TaN for copper fill, but this application will not be further discussed in any detail.
Several sputtering procedures are known for accomplishing a nearly conformal coating of metal into high aspect-ratio holes. Unfortunately, most of them tend to involve expensive equipment or require excessive operating times because of the slow effective deposition rates in high-aspect ratio holes. However, a recently developed sputtering technology, called SIP+ and developed by Applied Materials, Inc. of Santa Clara, Calif., offers many advantages for coating metal, particularly copper, into high aspect-ratio holes and in integrated circuit structures greatly desired for advanced electronics. SIP+ is a modification of SIP, which stands for self-ionized plasma. Fu et al. describes a version of SIP in U.S. patent application Ser. No. 09/373,097, filed Aug. 12, 1999 and now issued as U.S. Pat. No. 6,183,614. In SIP, various modifications are made to a standard DC magnetron sputter reactor to achieve a high-density metal plasma and to guide the metal ions to the wafer. Some of the techniques include high target power, in particular, high effective target power densities in the area of the magnetron, and small but strong magnets scanned about the back of the generally planar target. The design of the magnetron is also critical. These techniques increase the plasma density adjacent to the sputter target with the result that a significant fraction of the sputtered atoms become ionized. Two principal effects are obtained. First, the sputtered metal ions can act at least partially as the ions sputtering the target so that the pressure of argon typically used as a sputter working gas can be reduced. In the extreme case possible with copper, called sustained self-sputtering, the argon can be completely eliminated once the plasma has been excited. Secondly, various techniques can be used to guide and accelerate the metal ions to deep within the high aspect-ratio holes.
SIP+, as described by Fu et al. in U.S. patent application Ser. No. 09/490,026, filed Jan. 21, 2000 and now issued as U.S. Pat. No. 6,251,242, relies upon a target having a novel complex shape rather than the conventional planar shape used in SIP. Fu et al. describe subsequent developments of SIP+ in U.S. patent application Ser. No. 09/518,180, filed Mar. 2, 2000 and now issued as U.S. Pat. No. 6,277,249. The SIP+ target has a deep annular groove or vault formed in its side facing the substrate. Various magnet configurations are possible involving anti-parallel magnets placed on the sidewalls of the vault and/or a nested ring magnet placed over the vault roof and rotated along the path of the closed vault.
Nonetheless, further improvements are desired.
A self-ionized plasma requires large amounts of power to be applied to the target. The irregularly shaped vaulted target typically used in SIP+ tends to develop hot spots which are difficult to cool, particularly in the presence of the large number of magnets typically used.
Self-ionized plasmas, particularly with vaulted targets, require strong magnetic fields having somewhat complex shapes. Care must be taken to assure that the fields do not create very uneven erosion patterns in the target. Uniform erosion patterns are desired. Otherwise, the most heavily sputtered portion of the target may be eroded completely through while a large amount of target material remains in other regions of the target. Targets, particularly ones having the complex vaulted shape, are expensive to fabricate, and their lifetimes should be increased by a more uniform erosion pattern.
Although a high ionization fraction is often required for sputtering into high-aspect, holes, a complex device interconnect structure like dual damascene may require a more involved deposition sequence or different thickness of different portions of the deposited film.
Sputtering targets need to be occasionally replaced after they have been significantly eroded. Target replacement in SIP+ reactors is more difficult because the target tends to be relatively heavy, about 40 kg, and there are many magnets, some or all of which may be configured to rotate with associated motors and shafts, which factors increase the complexity, difficulty, and precision needed to change a target. Both the magnet design and chamber configuration need to facilitate the quick, economical replacement of the target.
According to one aspect of the invention, anti-parallel sets of magnets are disposed completely around the inner and outer sidewalls of an annular vault formed in the target, and a small nest of opposed magnets is rotated over the roof of the vault. Advantageously, the set of magnets disposed about for the outer sidewall is stationary and the set for the inner sidewall, although substantially circular, is rotated together with the roof magnets.
According to a further aspect of the invention, the inner vault sidewall magnets are divided into two axial parts of the same polarity which are separated by a non-magnetic spacer, thereby providing magnetic field over a larger area of the inner vault sidewall.
According to another aspect of the invention including the rotating center magnet, cooling liquid is pumped through the shaft upon which the inner sidewall magnets are supported and rotated and exits the shaft into a gap between the bottom of the inner sidewall magnets and the flattened portion of the target within the ring of stationary magnets surrounding the outer sidewall of the vault. Advantageously, the cooling water exits the space behind the target through apertures set in the outer vault sidewall near the bottom of the vault.
According to yet another aspect of the invention, the chamber wall surrounding the vaulted target and magnetron is composed of two parts. The vault shaped target is fixed to the bottom part of the chamber wall, while the magnets are supported on the top part of the chamber wall, which also supports the chamber lid and other parts associated with the rotating magnets. The cooling water port to the exterior is advantageously also on the top part. Thereby, when the target needs to be replaced, the entire upper assembly, including both upper chamber wall parts, is lifted from the chamber bottom. Then, the bottom chamber wall part and target are separated from the upper chamber wall part and associated other elements, and a replacement set of bottom chamber wall part and target are substituted.